Conventionally, it has been common to perform plastic working on a cold-rolled steel sheet to form a shape having predetermined dimensions, and thereafter perform Zn plating (post-Zn plating) to produce a component. However, for the recent automobile components, home electrical appliances and the like, for the purpose of enhancing corrosion resistance and durability of the component, and achieving cost reduction by omission of steps, it is increasingly the case that a Zn-based plated steel sheet obtained by coating Zn or a Zn alloy on a steel sheet is used as a raw material, and a component is produced by performing plastic working on the steel sheet.
As used herein, a steel sheet obtained by plating Zn or a Zn-containing alloy on the surface of a steel sheet is referred to as a Zn-based plated steel sheet.
Here, the plated layer of the Zn-based plated steel sheet is inferior in ductility to the underlying steel sheet, and therefore, cracks may occur in the plated layer when plastic working is performed on the plated steel sheet used as the raw material. In general, the cracks in the plated layer are more prominent in bulging processing in which a stronger tensile stress is likely to be exerted on the plated layer than in drawing processing. Then, when such cracks in the plated layer, or in other words, working cracks occur, the plated layer is divided. As a result, the underlying steel sheet is exposed from the gaps between the divided plated layers, which may lead to deterioration in the corrosion resistance of the workpiece. When the plated layer is a Zn-based plated layer and the degree of working cracks is slight, deterioration in the corrosion resistance is inconspicuous because of the sacrificial protection effect of the Zn-based plated layer even if the underlying steel sheet is exposed. However, when the degree of the working cracks is significant, red rust occurs from the exposed portion of the underlying steel sheet to degrade the external appearance, or corrosion advances from the exposed portion of the underlying steel sheet to reduce the thickness of the underlying steel sheet, which may cause a decrease in the strength of the workpiece.
Therefore, as a method for suppressing the deterioration in the corrosion resistance of the worked portion, it is possible to use, as a raw material, a Zn—Al—Mg-based plated steel sheet coated with a Zn—Al—Mg-based alloy having excellent corrosion resistance. However, this cannot prevent working cracks, so that it is difficult to prevent the occurrence of red rust.
As a processing method capable of suppressing working cracks in the plated layer, PTL 1 discloses a processing method in which a Zn-based plated steel sheet is heated and held in a temperature range of not less than 50° C. and not more than 150° C., to process the Zn-based plated steel sheet into a target shape. This processing method is intended to heat and hold the Zn-based plated steel sheet so as to apply processing to the plated layer in a state in which the ductility thereof is increased, thereby suppressing working cracks (cracks) in the plated layer.